Tiff Needell's 1980 Rover V8 S SD1 Group 2 on our rollers
Discussion
Mignon said:
227bhp said:
The obvious one is that two valves let more air in than one
The other I came up with was mixture motion in the cylinder,
The actual list of factors which this annoyingly short question (to ask anyway) requires coverage of include, volumetric efficiency, chamber turbulence, chamber scavenging, residual gas content, in cylinder flow patterns, pulse tuning, combustion efficiency, chamber size and shape, compression ratio, ignition timing, valve motion, cam lobe acceleration and frictional losses. Many of these factors are interlinked so explaining one means explaining another first or even both together because there's a feedback loop in there. It's a frigging nightmare trying to work out how best to even write it.The other I came up with was mixture motion in the cylinder,
It's like a flow chart or even a tree, the main core of the issue is the trunk, you then realise something relevant branches off over there so you explain that, then you realise it goes a bit further to another branch and then a twig and that is influenced by the leaves on the end. Then you realise you've practically written a bloody book on the topic and then your mind wanders down to where the roots are....
Mignon said:
However I'm afraid the one factor that is actually irrelevant in terms of how 4v engines produce torque per litre compared to 2v ones is that 4 valve cylinder heads let in more air. Sorry 
Haha, I would have put money on that being a major factor, you'll have to explain that one to us.Having made a start on this I've realised I'd vastly underestimated how much effort it would take, even knowing that it was a complex topic. It's taken 2 hours just to write part 1 and I think it might need 7 or more parts but anyway here's the introduction as a taster.
Why do 4v per cylinder engines produce more torque per litre than 2v per cylinder ones? - Part 1
To answer this question fully encompasses so many factors it covers most of four stroke engine tuning theory. That actually makes it one of the most important questions it's possible to ask about engines because if you understand this there's not much left.
Most engines, certainly most competition ones, are designed to produce the most horsepower possible because that's what actually propels a vehicle both in terms of acceleration and top speed. Simplistic quotes you see bandied about like "horsepower sells cars but torque wins races" are just gibberish from the mouths of people who don't actually understand physics. However it's certainly true that horsepower and torque are related and the more torque per litre you can obtain the lower an engine needs to rev to produce a given amount of power. That has important implications for reliability, frictional losses and the strength and cost of internal engine components.
Obtaining torque per litre is a function of three things, each of which has many sub factors influencing the totality.
1) Volumetric efficiency - getting as much air/fuel mix as possible into the cylinder per cycle.
2) Combustion efficiency - burning that air/fuel mix to obtain the greatest amount of work out of it.
3) Frictional losses - getting the work done to the piston all the way down to the flywheel and out to the transmission without losing any more of it than necessary.
Volumetric Efficiency
This is defined as the actual mass of air processed by the cylinder in a single stroke divided by the mass of air that the swept volume of the cylinder would contain at atmospheric pressure. Let's consider a cylinder with a compression ratio of 11:1. The swept volume is 10 units, the combustion chamber volume is 1 unit so the CR is SV + CV / CV so 10+1/1 = 11.
Imagine we have camshaft timing such that the inlet and exhaust valves are never open at the same time. At the end of the exhaust stroke the piston has certainly displaced all the burned exhaust gases in the swept volume but the combustion chamber volume is still full of spent gas. The best it would be possible to do is get those residual gases down to atmospheric pressure but they'll still be there at the start of the inlet stroke and they'll contaminate the fresh incoming charge. Even if we manage to operate the engine at 100% volumetric efficiency so the swept volume gets a full charge of fresh fuel/air mixture the net contents at the start of the compression stroke will be 10 units of fresh charge and 1 unit of spent gas.
Ideally we would like to displace all of the spent gas and get 11 units of fresh charge into the cylinder creating a volumetric efficiency of 110%. The only way to do that is to have the inlet and exhaust valves open at the same time in the hope that as fresh charge starts to flow into the cylinder it displaces the spent gas down the exhaust ports. This ploy is fraught with problems though.
If we take a 2v per cylinder engine with vertical valves, size the valves in the best possible ratio for maximum power and leave the minimum possible gap between each valve and the bore wall and between the inlet and exhaust valve themselves about the most we can achieve is an inlet valve of 28% of the bore area and an exhaust valve of about 18%. This means 46% of the bore area is filled with valve and 54% is dead area that needs to be scavenged. It's possible to exceed those numbers with inclined valves but that brings in other issues. It's not very realistic to assume that as that single inlet valve opens, the incoming charge is somehow going to sweep round all of the dead areas on both sides of the bore and herd those spent gases like errant sheep into the little exhaust valve that's waiting to receive them. You might as well be trying to herd cats.
However compare a 4v per cylinder engine. We can obtain about 38% of the bore area filled by inlet valve and 27% filled by exhaust valve with normal valve inclination angles for a total of 65% leaving only 35% dead area to be scavenged. Moreover the flow patterns and valve positions mean that each inlet valve scours its own side of the bore directing spent gas straight to the waiting exhaust valve next to it. This leads to a far more efficient scavenging process and much less chance of spent gases contaminating the incoming charge.
Of course the issue is not as simplistic as that for real engines. In practice with good design we're mainly trying to scavenge the combustion chamber not the entire bore area. For 2v engines there are a multiplicity of possible layouts. Vertical valves and inclined ones, bath tub shaped chambers, heart shaped ones, hemispherical or even flat topped heads and bowl in piston layouts. Some are appalling from a scavenging point of view and some are good but as a general rule they all get better with increasing compression ratio and a more compact chamber. However the basic 4v layout has great scavenging even at low compression ratios and doesn't get markedly better at high ones. In fact high CR can bring its own problems in this regard with 4v engines as piston domes intrude into the cylinder head space but we'll look at this in more detail under combustion efficiency.
So it's not the larger valve area or total flow potential that gives the 4v engine design its greatest advantage in terms of torque per litre over the 2v one, it's scavenging. Even on low compression stock road engines it's the single biggest factor in why 4v engines produce between 10% and 14% more torque per litre than 2v ones because they operate at higher volumetric efficiency with less residual gas. With high compression race engines and best design the equation starts to change. 2v engines get much better and 4v ones don't so much so some of the advantage gets clawed back.
There are however a multitude of other factors that affect the VE issue and we'll look at more of those in part 2.
Why do 4v per cylinder engines produce more torque per litre than 2v per cylinder ones? - Part 1
To answer this question fully encompasses so many factors it covers most of four stroke engine tuning theory. That actually makes it one of the most important questions it's possible to ask about engines because if you understand this there's not much left.
Most engines, certainly most competition ones, are designed to produce the most horsepower possible because that's what actually propels a vehicle both in terms of acceleration and top speed. Simplistic quotes you see bandied about like "horsepower sells cars but torque wins races" are just gibberish from the mouths of people who don't actually understand physics. However it's certainly true that horsepower and torque are related and the more torque per litre you can obtain the lower an engine needs to rev to produce a given amount of power. That has important implications for reliability, frictional losses and the strength and cost of internal engine components.
Obtaining torque per litre is a function of three things, each of which has many sub factors influencing the totality.
1) Volumetric efficiency - getting as much air/fuel mix as possible into the cylinder per cycle.
2) Combustion efficiency - burning that air/fuel mix to obtain the greatest amount of work out of it.
3) Frictional losses - getting the work done to the piston all the way down to the flywheel and out to the transmission without losing any more of it than necessary.
Volumetric Efficiency
This is defined as the actual mass of air processed by the cylinder in a single stroke divided by the mass of air that the swept volume of the cylinder would contain at atmospheric pressure. Let's consider a cylinder with a compression ratio of 11:1. The swept volume is 10 units, the combustion chamber volume is 1 unit so the CR is SV + CV / CV so 10+1/1 = 11.
Imagine we have camshaft timing such that the inlet and exhaust valves are never open at the same time. At the end of the exhaust stroke the piston has certainly displaced all the burned exhaust gases in the swept volume but the combustion chamber volume is still full of spent gas. The best it would be possible to do is get those residual gases down to atmospheric pressure but they'll still be there at the start of the inlet stroke and they'll contaminate the fresh incoming charge. Even if we manage to operate the engine at 100% volumetric efficiency so the swept volume gets a full charge of fresh fuel/air mixture the net contents at the start of the compression stroke will be 10 units of fresh charge and 1 unit of spent gas.
Ideally we would like to displace all of the spent gas and get 11 units of fresh charge into the cylinder creating a volumetric efficiency of 110%. The only way to do that is to have the inlet and exhaust valves open at the same time in the hope that as fresh charge starts to flow into the cylinder it displaces the spent gas down the exhaust ports. This ploy is fraught with problems though.
If we take a 2v per cylinder engine with vertical valves, size the valves in the best possible ratio for maximum power and leave the minimum possible gap between each valve and the bore wall and between the inlet and exhaust valve themselves about the most we can achieve is an inlet valve of 28% of the bore area and an exhaust valve of about 18%. This means 46% of the bore area is filled with valve and 54% is dead area that needs to be scavenged. It's possible to exceed those numbers with inclined valves but that brings in other issues. It's not very realistic to assume that as that single inlet valve opens, the incoming charge is somehow going to sweep round all of the dead areas on both sides of the bore and herd those spent gases like errant sheep into the little exhaust valve that's waiting to receive them. You might as well be trying to herd cats.
However compare a 4v per cylinder engine. We can obtain about 38% of the bore area filled by inlet valve and 27% filled by exhaust valve with normal valve inclination angles for a total of 65% leaving only 35% dead area to be scavenged. Moreover the flow patterns and valve positions mean that each inlet valve scours its own side of the bore directing spent gas straight to the waiting exhaust valve next to it. This leads to a far more efficient scavenging process and much less chance of spent gases contaminating the incoming charge.
Of course the issue is not as simplistic as that for real engines. In practice with good design we're mainly trying to scavenge the combustion chamber not the entire bore area. For 2v engines there are a multiplicity of possible layouts. Vertical valves and inclined ones, bath tub shaped chambers, heart shaped ones, hemispherical or even flat topped heads and bowl in piston layouts. Some are appalling from a scavenging point of view and some are good but as a general rule they all get better with increasing compression ratio and a more compact chamber. However the basic 4v layout has great scavenging even at low compression ratios and doesn't get markedly better at high ones. In fact high CR can bring its own problems in this regard with 4v engines as piston domes intrude into the cylinder head space but we'll look at this in more detail under combustion efficiency.
So it's not the larger valve area or total flow potential that gives the 4v engine design its greatest advantage in terms of torque per litre over the 2v one, it's scavenging. Even on low compression stock road engines it's the single biggest factor in why 4v engines produce between 10% and 14% more torque per litre than 2v ones because they operate at higher volumetric efficiency with less residual gas. With high compression race engines and best design the equation starts to change. 2v engines get much better and 4v ones don't so much so some of the advantage gets clawed back.
There are however a multitude of other factors that affect the VE issue and we'll look at more of those in part 2.
Mignon said:
However I'm afraid the one factor that is actually irrelevant in terms of how 4v engines produce torque per litre compared to 2v ones is that 4 valve cylinder heads let in more air.
Well, that's a long-help misconception dashed.Is it still the case that in practice, bigger (valves) is still better (engine breathing) for 2V? I have always assumed that a better flowing intake will mean less need to extend the valve duration to get volumetric efficiency up and hence less at the mercy of pulse tuning effects.
GreenV8S said:
Well, that's a long-help misconception dashed.
Is it still the case that in practice, bigger (valves) is still better (engine breathing) for 2V? I have always assumed that a better flowing intake will mean less need to extend the valve duration to get volumetric efficiency up and hence less at the mercy of pulse tuning effects.
Total flow determines power potential i.e at what rpm the engine can still breathe efficiently. It doesn't much affect how much air the engine can take in at a single gulp though. When the cylinders are full they're full and that's determined by scavenging and pulse tuning.Is it still the case that in practice, bigger (valves) is still better (engine breathing) for 2V? I have always assumed that a better flowing intake will mean less need to extend the valve duration to get volumetric efficiency up and hence less at the mercy of pulse tuning effects.
Part 2
It's commonly, and erroneously, thought that engines can only operate at best at 100% volumetric efficiency. Nothing could be further from the truth. We can utilise pressure waves in the inlet and exhaust tracts to force more air into the cylinder from the inlet port, extract residual gases from the chamber using low pressure pulses at the exhaust valve and even push back some of the incoming charge that made it all the way across the chamber and down the exhaust port with high pressure waves. Pressure pulses are created when the valves open. They resonate up and down the pipework and reflect off changes in area and junctions. The strongest pulses are obtained with long duration camshafts and fast opening valves. In fact ideally from both the flow and the pulse tuning point of view we would like the valves to open fully instantaneously but that's of course not possible. But it is certainly true that other things being equal we can open smaller valves faster than bigger ones. As such the 4v engine with its smaller and lighter valves has advantages there. It's not a huge factor but it can potentially allow slightly better pulse tuning and consequently better VE.
With optimum pulse tuning 4v race engines can operate at about 135% VE. That means we're filling the cylinders with fresh charge over not just all of the swept volume but also the combustion chamber space and then about another 20% over and above atmospheric pressure on top of all that.
The next major factor which differentiates how 4v engines work compared to 2v ones is a matter of geometry, more specifically the ratio of valve circumference to valve area. If we take two engines of identical bore and stroke and the same total valve area, which is the primary determinent of total flow and power potential, then the 4v engine has 1.4 times as much valve circumference as the 2v one. A single 45mm valve has a total area of 1590 sq mm and a total circumference of 141 mm. Two 31.8mm valves have the same area but a total circumference of 200 mm. This gives the 4v engine much more low lift flow compared to its peak flow than the 2v engine which in turn affects how the engine operates at low lifts during the valve overlap period when scavenging and pulse tuning is taking place. In short it means that whatever pressure pulses we manage to generate with carefully selected inlet and exhaust lengths have a greater valve open area and flow potential to operate with for a given camshaft timing and valve open amount.
This in turn means that 4v engines can generate the same bhp as 2v ones with shorter cam timing which gives a better spread of torque than a long duration cam will show.
Finally on VE it's another common misconception that the total flow advantage, due to the greater valve area, of the 4v engine produces more torque per litre. In fact total flow primarily determines power potential not torque potential. Torque potential is mainly affected by scavenging and pulse tuning as well as combustion efficiency considerations which we'll consider next.
It's commonly, and erroneously, thought that engines can only operate at best at 100% volumetric efficiency. Nothing could be further from the truth. We can utilise pressure waves in the inlet and exhaust tracts to force more air into the cylinder from the inlet port, extract residual gases from the chamber using low pressure pulses at the exhaust valve and even push back some of the incoming charge that made it all the way across the chamber and down the exhaust port with high pressure waves. Pressure pulses are created when the valves open. They resonate up and down the pipework and reflect off changes in area and junctions. The strongest pulses are obtained with long duration camshafts and fast opening valves. In fact ideally from both the flow and the pulse tuning point of view we would like the valves to open fully instantaneously but that's of course not possible. But it is certainly true that other things being equal we can open smaller valves faster than bigger ones. As such the 4v engine with its smaller and lighter valves has advantages there. It's not a huge factor but it can potentially allow slightly better pulse tuning and consequently better VE.
With optimum pulse tuning 4v race engines can operate at about 135% VE. That means we're filling the cylinders with fresh charge over not just all of the swept volume but also the combustion chamber space and then about another 20% over and above atmospheric pressure on top of all that.
The next major factor which differentiates how 4v engines work compared to 2v ones is a matter of geometry, more specifically the ratio of valve circumference to valve area. If we take two engines of identical bore and stroke and the same total valve area, which is the primary determinent of total flow and power potential, then the 4v engine has 1.4 times as much valve circumference as the 2v one. A single 45mm valve has a total area of 1590 sq mm and a total circumference of 141 mm. Two 31.8mm valves have the same area but a total circumference of 200 mm. This gives the 4v engine much more low lift flow compared to its peak flow than the 2v engine which in turn affects how the engine operates at low lifts during the valve overlap period when scavenging and pulse tuning is taking place. In short it means that whatever pressure pulses we manage to generate with carefully selected inlet and exhaust lengths have a greater valve open area and flow potential to operate with for a given camshaft timing and valve open amount.
This in turn means that 4v engines can generate the same bhp as 2v ones with shorter cam timing which gives a better spread of torque than a long duration cam will show.
Finally on VE it's another common misconception that the total flow advantage, due to the greater valve area, of the 4v engine produces more torque per litre. In fact total flow primarily determines power potential not torque potential. Torque potential is mainly affected by scavenging and pulse tuning as well as combustion efficiency considerations which we'll consider next.
AW111 said:
I would be interested also.
OT -What happened to 5 valve heads?
Yamaha used them on bikes, and built 20v heads for Toyota; Ferrari used them : why did they come into favour then disappear?
They have a greater inlet flow potential although not by much (about 5%) but they have huge problems with getting big enough tappets into the space available to open the valves fast and high. The tappet diameter determines the maximum valve opening rate which in turn determines the peak lift in a given duration. When you plough through all the equations they add a huge amount of complexity and cost, mean you have to use strange cam geometry because the centre tappet on the inlet side has to be offset from the other two and then often you end up with less power. I wrote about this in respect of the VW 5v V6 quad cam engine many years ago which had good flow but ended up having to use 24mm diameter tappets when most 4v twin OHC engines have space for 35 mm tappets or even more. It was a stupid idea from the get go but marketing considerations ruled the decision not engineering ones. It had a high tech appeal but it was actually crap.OT -What happened to 5 valve heads?
Yamaha used them on bikes, and built 20v heads for Toyota; Ferrari used them : why did they come into favour then disappear?
Ok Gentlemen. 1st time I've posted anything on this site, but I guess the ongoing rage and speculation about my car caused me some amusement. Purchased about 4 years ago from Simon Hope H&H auctions by me as a ' did not sell car '. The engine was indeed advertised as a TVR engine with a huge power output and dry sump. Believe me it didn't have that engine in it at that time as it was basically a knackered 3500 from a old rover producing about 100bhp on a good day with no dry sump. In fact it caught fire on the Bugatti circuit at Le Mans the 1st time I drove it. I suspect the TVR engine had been removed and the flash looking carps bolted onto a snotter of an engine to get it sold. Enter JE engineering with a specific instruction by me to return the car to 3500 spec, with a new engine and a standard firing order and crank , as I race a lot with various cars and have a disliking for people cheating on track. The finished car was then became a reliability nightmare throwing its dry sump drive belts with alarming regularity resulting in JE rebuilding the engine at least 3 times. Getting fed up with this ongoing saga of on track embarrassment I entrusted the engine to Blaine Neaves a well known MGB engine chap who stripped the engine, balanced everything and confirmed its 3500 capacity. As an engine builder himself he was absolutely confident that this engine would only produce about 260bhp as it isn't one of his engines and is running a standard crank. JE had previously confirmed a power output of 350bhp but Blaine insisted that simply wasn't going to be the case. To prove this he took it to his trusted dyno chaps. Much to his surprise and to be fair mine , the power curves are correct. The above is fact so any doubting will amuse me yet further. Incidentally the ' troll ' you referred to ' Guy ' did indeed rebuild all of the suspension and is indeed one of the drivers who will be sharing the car the car with me on circuit this year. As far as proving its capacity , whilst not being necessary, I welcome the FIA inspection.
GreenV8S said:
Hasn't that already been provided in the first post of this thread?
Clearly not. We were told in the first post it had been engine dynoed at 377 bhp but no mention of torque. Now we are told the engine dyno figure was actually 350 bhp but again no mention of torque. 100 bhp per litre for a 2v is no sort of surprise to any competent engine builder but torque per litre of over 80 ft lbs is.Edit Ooooooh hang on. Are we saying that it's Peter's rolling road figure that is the confirmation of the previous engine dyno claim? i.e Peter is the trusted dyno guy referred to? So we're all the way back to where we started and no wiser. Jeez.
Edited by Mignon on Monday 27th February 18:24
Hi Peter. Yes I think people liked looking at it as its been missing for some time. As you are aware I race all sorts of cars from F1 to vintage stuff throughout Europe and I was warned that these sites encourage entrenched opinion..which is why I as a consequence try and steer well clear. I was hoping a little input of cold facts would assist from someone who actually races these things. Correct me if I'm wrong but the engine developed 322 BHP at the wheels , about 354 BHP at the flywheel at 5950 rpm . Torque seems to be 302 ft-lbs at 5500 rpm from what I can work out. To be fair I just enjoy the cars for what they are and try not to get bogged down in who's right and who's wrong as it seems to be rather emotive.Oh and yes it is two valves per head for the purposes of clarification. Cheers Martin
Who said all publicity is good publicity Martin?
The wheel power was 322 and coastdown losses 28 giving our guesstimate of 350 at engine. Close to JE 350? I had been told it had seen 377 on JE dyno but that may have been incorrect. Like you I just enjoy driving things and listening to things. I have never claimed our figures are Gospel, neither do I care as long as figures are repeatable I can then say they are comparable with other cars on my rollers. Yours is a flying machine, better than the flat plane ex Tony Pond TR8 we tested years ago! I posted the car details as I thought it nice to show folk a lovely car that had been awol for a while, there cannot be many MK1 SD1s left at all, let alone character cars like yours! I know it pales into insignificance compared to some of your stable such as Porsche 862 Le Mans car or your (James Bond's original car?) blown 4 1/2 Bentley but it has a place in my heart for what it is, a much respected SD1
Peter
The wheel power was 322 and coastdown losses 28 giving our guesstimate of 350 at engine. Close to JE 350? I had been told it had seen 377 on JE dyno but that may have been incorrect. Like you I just enjoy driving things and listening to things. I have never claimed our figures are Gospel, neither do I care as long as figures are repeatable I can then say they are comparable with other cars on my rollers. Yours is a flying machine, better than the flat plane ex Tony Pond TR8 we tested years ago! I posted the car details as I thought it nice to show folk a lovely car that had been awol for a while, there cannot be many MK1 SD1s left at all, let alone character cars like yours! I know it pales into insignificance compared to some of your stable such as Porsche 862 Le Mans car or your (James Bond's original car?) blown 4 1/2 Bentley but it has a place in my heart for what it is, a much respected SD1
Peter
Hi Peter yes your readings from what I'm hearing in the industry are generally recognised as the most accurate and its good to hear that your a real petrol head. I know where Ill be coming in the future when something needs checking. Say hello on a track somewhere or better still come to Donnington and jump in the passenger seat. It would be my pleasure.Guy will be there too so you can meet the ' troll ' Funny that as whoever wrote that has obviously seen a picture of him..!! Cheers
I'm enjoying the technical aspects to this thread - informative stuff, please keep it coming.
To someone who knows very little, does the fact that this car (the subject of some division of opinion) has a dry sump make any difference to the torque per litre output?
Would the dry sump pump replace the OEM oil pump in the block? I ask as I am sure I have read before that Nascar V8's ran/still run dry sumps with very aggressive scavenging stages, the idea being to reduce the overall pressure in the crankcase. I wonder if this would fit in with Mignon's posts yet to be made about frictional/parasitic losses. I am sure there would of course be a certain amount of energy taken to drive the dry sump pump, but I have no idea of the relative benefits of either OEM or dry sump setup. I look forward to finding out.
To someone who knows very little, does the fact that this car (the subject of some division of opinion) has a dry sump make any difference to the torque per litre output?
Would the dry sump pump replace the OEM oil pump in the block? I ask as I am sure I have read before that Nascar V8's ran/still run dry sumps with very aggressive scavenging stages, the idea being to reduce the overall pressure in the crankcase. I wonder if this would fit in with Mignon's posts yet to be made about frictional/parasitic losses. I am sure there would of course be a certain amount of energy taken to drive the dry sump pump, but I have no idea of the relative benefits of either OEM or dry sump setup. I look forward to finding out.
Part 3
Combustion Efficiency
This part of the topic covers everything that affects how much power is released from the fuel burned. Factors affecting that include burn speed, ignition advance, mixture quality, in-cylinder turbulence, residual gas fraction and compression ratio.
In an ideal engine we would like all the fuel/air mix to ignite simultaneously at TDC and expend all of its energy driving the piston down on the power stroke. In reality such an event would be an explosion and the engine would never be able to withstand it. Actual combustion takes a finite amount of time and even though this is very fast in human terms, a couple of milliseconds, it's a considerable number of crankshaft degrees in a high rpm engine. Because of this we need to ignite the mixture well before TDC so that the main part of it burns just after TDC. Optimising ignition advance for a specific engine is just a matter of experimenting until the best torque is obtained at each rpm but it just so works out that in almost every engine this creates a situation where approximately 50% of the fuel/air mixture has burned by about 10 to 15 degrees after TDC and this is also the position where peak cylinder pressure is generated.
However all of the fuel that burns while the piston is still rising on the compression stroke does negative work on the engine. It detracts from the power produced on the down stroke following that. What that tells us is that an engine with a slower burn which requires more ignition advance will produce less net power from the same amount of fuel/air as a faster burning engine requiring less ignition advance. Burn speed is primarily determined by chamber size and shape but other important factors are spark plug position, volumetric efficiency, residual gas fraction, compression ratio and turbulence.
As already stated, 2v engines have a multiplicity of different configurations. Compact combustion chambers burn quickly and efficiently but open chambers like hemi heads and those with no combustion chamber in the actual head where the piston runs well down the bore burn much more slowly. The best 2v road engines might require as little as 30 degrees total ignition advance or even in the high 20s for small bore engines but the worst can need more than 40 degrees of advance. The constraints of placing 4 valves in a cylinder mean that all 4v engines are very similar in terms of chamber shape with a convenient position right at the centre where the spark plug can go. In a 2v engine the plug must always be off to one side somewhere.
When the spark plug fires it ignites a few molecules of fuel near the plug gap. Those burning molecules get carried round the chamber setting light to other molecules by the turbulence that exists in all engines. The process starts fairly slowly and for about 15 to 20 crank degrees not much happens but the flame front expands exponentially at first, reaches a steady state for a while until most of the mixture is alight and then tapers off again. The percentage of the initial fuel/air mix that has burned in a given number of crankshaft degrees after the plug fires can be calculated by the Wiebe equation but it's not necessary to go into details of that here. What we are more concerned with is the differences between 4v and 2v engines.
Factors which speed up the burn rate are central spark plug position, high VE, low residual gas fraction and high turbulence. In every one of these the 4v engine scores highly over the 2v one. As a consequence of this 4v engines can require as little as low to mid 20s degrees of ignition advance at WOT and high rpm.
Turbulence and in-cylinder flow patterns are very different between 4v and 2v engines. The single inlet stream on 2v engines tends to travel horizontally round the chamber when the piston is near TDC and is usually called swirl. The twin inlet streams on 4v engines travel across the piston crown, back up the far side of the chamber and across the top of it in a pattern called tumble. 4v turbulence is more pronounced than 2v and tends to persist longer after the piston rises to TDC compressing the incoming charge hence leading to more rapid flame propagation.
By measuring in-cylinder pressure over time and comparing that with the theoretical optimum pressure from an instantaneous burn at TDC we can calculate the power loss that ignition advance creates. In an engine requiring 30 degrees of advance about 10% of the engine's potential power is wasted in negative work on the compression stroke. That would represent a very good result for a 2v engine. 40 degrees of advance creates a 15% or greater loss. Low to mid 20 degrees of advance involves about 5% loss. On average a good 4v road engine therefore has about 5% advantage in combustion efficiency over even a fast burning 2v one just due to burn speed before we take into account compression ratio which is the next thing to consider and we'll also start to look at how highly tuned race engines differ from the road ones discussed so far.
Combustion Efficiency
This part of the topic covers everything that affects how much power is released from the fuel burned. Factors affecting that include burn speed, ignition advance, mixture quality, in-cylinder turbulence, residual gas fraction and compression ratio.
In an ideal engine we would like all the fuel/air mix to ignite simultaneously at TDC and expend all of its energy driving the piston down on the power stroke. In reality such an event would be an explosion and the engine would never be able to withstand it. Actual combustion takes a finite amount of time and even though this is very fast in human terms, a couple of milliseconds, it's a considerable number of crankshaft degrees in a high rpm engine. Because of this we need to ignite the mixture well before TDC so that the main part of it burns just after TDC. Optimising ignition advance for a specific engine is just a matter of experimenting until the best torque is obtained at each rpm but it just so works out that in almost every engine this creates a situation where approximately 50% of the fuel/air mixture has burned by about 10 to 15 degrees after TDC and this is also the position where peak cylinder pressure is generated.
However all of the fuel that burns while the piston is still rising on the compression stroke does negative work on the engine. It detracts from the power produced on the down stroke following that. What that tells us is that an engine with a slower burn which requires more ignition advance will produce less net power from the same amount of fuel/air as a faster burning engine requiring less ignition advance. Burn speed is primarily determined by chamber size and shape but other important factors are spark plug position, volumetric efficiency, residual gas fraction, compression ratio and turbulence.
As already stated, 2v engines have a multiplicity of different configurations. Compact combustion chambers burn quickly and efficiently but open chambers like hemi heads and those with no combustion chamber in the actual head where the piston runs well down the bore burn much more slowly. The best 2v road engines might require as little as 30 degrees total ignition advance or even in the high 20s for small bore engines but the worst can need more than 40 degrees of advance. The constraints of placing 4 valves in a cylinder mean that all 4v engines are very similar in terms of chamber shape with a convenient position right at the centre where the spark plug can go. In a 2v engine the plug must always be off to one side somewhere.
When the spark plug fires it ignites a few molecules of fuel near the plug gap. Those burning molecules get carried round the chamber setting light to other molecules by the turbulence that exists in all engines. The process starts fairly slowly and for about 15 to 20 crank degrees not much happens but the flame front expands exponentially at first, reaches a steady state for a while until most of the mixture is alight and then tapers off again. The percentage of the initial fuel/air mix that has burned in a given number of crankshaft degrees after the plug fires can be calculated by the Wiebe equation but it's not necessary to go into details of that here. What we are more concerned with is the differences between 4v and 2v engines.
Factors which speed up the burn rate are central spark plug position, high VE, low residual gas fraction and high turbulence. In every one of these the 4v engine scores highly over the 2v one. As a consequence of this 4v engines can require as little as low to mid 20s degrees of ignition advance at WOT and high rpm.
Turbulence and in-cylinder flow patterns are very different between 4v and 2v engines. The single inlet stream on 2v engines tends to travel horizontally round the chamber when the piston is near TDC and is usually called swirl. The twin inlet streams on 4v engines travel across the piston crown, back up the far side of the chamber and across the top of it in a pattern called tumble. 4v turbulence is more pronounced than 2v and tends to persist longer after the piston rises to TDC compressing the incoming charge hence leading to more rapid flame propagation.
By measuring in-cylinder pressure over time and comparing that with the theoretical optimum pressure from an instantaneous burn at TDC we can calculate the power loss that ignition advance creates. In an engine requiring 30 degrees of advance about 10% of the engine's potential power is wasted in negative work on the compression stroke. That would represent a very good result for a 2v engine. 40 degrees of advance creates a 15% or greater loss. Low to mid 20 degrees of advance involves about 5% loss. On average a good 4v road engine therefore has about 5% advantage in combustion efficiency over even a fast burning 2v one just due to burn speed before we take into account compression ratio which is the next thing to consider and we'll also start to look at how highly tuned race engines differ from the road ones discussed so far.
Edited by Mignon on Wednesday 1st March 08:32
That is interesting about timing advance, as you probably know many folk think the more the advance the more the power!
I assume you write from theory rather than hard practice as we regularly find we run low 20s in our best 2v engines. Best A series 1380 two engines at 21 degrees, 1500 Spitfire/Midget based engines 22 degrees max advance. TR6 6 cylinder, best engine 22 degrees. Best 1840 B series 24 degrees, however run the same B with a milder cam more suited to 1840 than 1950 and advance needed was 25.5 degrees even though it made a wider spread of torque and power with the same peak toque and power. Standard race MGB engines with loosening restrictions over the years to make racing more interesting for spectators by increasing power have changed from 34 degrees max with 92 at the wheels winning in 1987 to 27 degrees and 120 at the wheels winning in 2016. These were all on Shell best pump fuel and booster.
The above engiines have smaller to larger combustion chambers in the order written.
Peter
I assume you write from theory rather than hard practice as we regularly find we run low 20s in our best 2v engines. Best A series 1380 two engines at 21 degrees, 1500 Spitfire/Midget based engines 22 degrees max advance. TR6 6 cylinder, best engine 22 degrees. Best 1840 B series 24 degrees, however run the same B with a milder cam more suited to 1840 than 1950 and advance needed was 25.5 degrees even though it made a wider spread of torque and power with the same peak toque and power. Standard race MGB engines with loosening restrictions over the years to make racing more interesting for spectators by increasing power have changed from 34 degrees max with 92 at the wheels winning in 1987 to 27 degrees and 120 at the wheels winning in 2016. These were all on Shell best pump fuel and booster.
The above engiines have smaller to larger combustion chambers in the order written.
Peter
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